1. Field of the Invention
The present invention relates to a demodulator for communication system. More particularly, the present invention relates to a non-coherent Frequency Shift Key (non-coherent FSK) demodulating circuit and method.
2. Description of Related Art
It is well known in wireless communication systems that in order to transmit information, one must modulate a sine wave with the information therewith. A way to modulate a sine wave includes frequency modulation (FM), which is most commonly used in FM radio, as well as other wireless devices such as pagers, cellular phones, cordless telephones etc. Among the various forms of frequency modulation used for data communications includes the frequency shift key (FSK) modulation. In FSK modulation, digital binary data is encoded through the modulation between two frequencies ƒ1 and ƒ0, known as mark (representing logic high level ‘1’), and space (representing logic low level ‘0’), and then transmitted therewith. The FSK modulated waveform is transmitted by a transmitter and received by a receiver whereby the transmitted FSK signal is demodulated and the digital binary data is restored.
FIG. 1, shows a block diagram of a conventional FSK receiver. The FSK receiver 100 includes a receiving antenna 110, a low noise amplifier (LNA) 120, a mixer 130, a low-pass filter 140, an analog-to-digital converter 150 and a demodulator 160. The antenna 110 receives a Radio Frequency (RF) signal transmitted by a transmitter. The RF signal is amplified by the low noise amplifier 120 and then is mixed with a local oscillating (LO) signal ƒc with a LO frequency by the mixer 130. Following, the low pass filter 140 gets rid of distortions from mixing the RF signal in the digital data-bearing signal. The filtered mixed RF signal is then sampled by the analog-to-digital converter 150, thereby a digital non-coherent frequency shift key signal DNFSK is obtained. The digital non-coherent frequency shift key signal DNFSK having the first frequency ƒ1 or the second frequency ƒ0 will be further demodulated by the demodulator 160 to obtain the digital signal transmitted by the transmitter modulated in the RF signal.
Currently, there are at least two kinds of demodulator 160 being applied in the FSK receiver 100 as shown in FIG. 2 and FIG. 3 respectively. Referring to FIG. 2, the block diagram shows a conventional demodulator using a correlation receiver. Whereas FIG. 3, shows the block diagram of a conventional demodulator using a discrimination detector.
In FIG. 2, the demodulator 160 includes a first correlator 210, a second correlator 220 and a comparator 230. An integral circuit is respectively used in the first correlator 210 and the second correlator 220 for calculating a correlation value of the digital non-coherent frequency shift key signal DNFSK, which has the first frequency ƒ1 or the second frequency ƒ0, respectively, representing logic high level or logic low level. The comparator 230 compares the correlation values that are output from the first correlator 210 and the second correlator 220, in order to obtain the digital signal D from the RF signal.
Referring to FIG. 3, the demodulator 160 includes a discriminator 310 and a decision logic 320. A differential circuit is used in the discriminator 310 for calculating the digital non-coherent frequency shift key signal DNFSK, which has the first frequency ƒ1 or the second frequency ƒ0, in time domain to obtain a differential value. The differential value is proportional to the first frequency ƒ1 and the second frequency ƒ0. The decision logic 320 thus obtains the digital signal D in the RF signal by judging the differential value that is output from the discriminator 310.
It is obvious that the conventional demodulator 160 requires a complicated circuit for calculating and obtaining the digital signal, as exemplified by the integral circuit in the demodulator of FIG. 2 or the differential circuit in the demodulator of FIG. 3. Therefore, there is a need of a simpler architecture and implementation compared to the above stated conventional demodulators.